1. Field of the Invention
The present invention generally relates to a process for producing a powder of perovskite-type double oxide generally represented by the formula ABO.sub.3, particularly to a process for producing the powder which has not only high perovskite ratio, but also homogeneous component and fine particles, moreover, which is excellent in degree of sintering.
2. Description of the Relevant Art
The perovskite-type double oxide has been extensively used for functional ceramics; such as, piezoelectrics, dielectrics, semiconductor, and material for a sensor or the like. Recently, it is actively promoted to improve the function and to add a function newly, wherein a raw powder is required as a homogeneous component, of fine particles and the property of being sintered at low temperature or the like.
Up to now, a solid phase synthesis and coprecipitation method have been used as a process for producing the powder of perovskite-type double oxide.
A summary of these methods is as follows.
(a) Solid phase synthesis
This method includes the steps of weighting compounds of each raw ingredient, mixing, and calcining. Although this method is generally used, high temperature reaction causes evaporation of the lead ingredient and heterogeneous component. And it is difficult to obtain fine particles.
In a reaction system including Nb, it is difficult to obtain a pure perovskite phase through the usual way, and the perovskite phase is contaminated with a pyrochlore phase. Since the pyrochlore phase causes it to lose degree of sintering, and make the electric property of sintered compact worse, several methods of decreasing pyrochlore phase is proved.
For example, a process for producing Pb(Mg.sub.1/3 Nb.sub.2/3)O.sub.3 is reported by S. L. Swartz, T. R. Shrout et al. (Mater. Res. Bull. 17,1245 (1982)). According to this production process, the pure perovskite phase is obtained by means of two steps of calcining, as follows;
MgO+Nb.sub.2 O.sub.5 .fwdarw.MgNb.sub.2 O.sub.6 (calcining temperature is about 1000.degree. C.)
1/3MgNb.sub.2 O.sub.6 +PbO.fwdarw.Pb(Mg.sub.1/3 N.sub.2/3)O.sub.3 (calcining temperature is about 900.degree. C.)
Although, in the usual single step of calcining, repetition of calcining and pulverizing achieves an increase in the ratio of perovskite phase, it is difficult to obtain pure perovskite phase by means of this process. In any event, this method has still had subjects in terms of particle size and homogeneity of compound.
(b) Coprecipitation method
This method includes the steps of preparing mixed solution of each ingredient, obtaining precipitate by means of reaction of said mixed solution and alkaline solution, drying of said precipitation and calcining.
In this process, it is also difficult to obtain pure perovskite phase. Even though the powder of pure perovskite phase is obtained, its degree of sintering is not good, because secondary particles form in the step of precipitating, drying and sintering.
As is mentioned above, the powder of lead group perovskite double oxide containing Nb obtained by means of said method is not pure perovskite phase and is not good in the degree of sintering. Therefore, it has been required to improve the method.